Composite materials and devices comprising single crystal silicon carbide heated by electromagnetic radiation

ABSTRACT

A composite material that increases in temperature upon exposure to electromagnetic radiation includes single crystal silicon carbide whiskers and fibrils in a matrix material. Also disclosed are heat-generating objects that include the composite material, and a method of generating heat.

CROSS-REFERENCE TO RELATED APPLICATIONS

This present application is a continuation-in-part of U.S. applicationSer. No. 11/392,612, filed on Mar. 30, 2006 and the contents therein areincorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to composite materials that quickly andefficiently increase temperature by absorption of electromagneticradiation. The invention also relates to devices comprising thecomposite materials, and to a method of heating using these devices.

BACKGROUND OF THE INVENTION

Many materials are known to not absorb microwave energy or otherelectromagnetic radiation. Such materials may be reflective ortransparent to the electromagnetic radiation without being affectedthereby. Therefore, these materials do not heat when exposed tomicrowave or radiowave fields.

Many materials are known to absorb electromagnetic radiation andtherefore will heat. Heating with microwave energy is but one example ofthis phenomenon, and many compositions that heat upon absorption ofmicrowave energy are known. For example, water, fats, and certain foodproducts absorb microwave energy and are heated thereby. Similarly,inorganic compounds such as carborundum powder, ferrites, zinc oxide,silicon carbide, and even carbon particles are known to heat uponabsorption of microwave energy. Such compounds can be used to impartheat to their surroundings.

However, not every form of such materials can be used in this way. Forexample, metal powder can be used to absorb microwave energy, and isused in combination with other compositions to form heatable objects.However, metal is highly conductive, and this high conductivity can leadto arcing or sparking. For example, a large mass of solid metaltypically cannot be placed in a microwave oven without causing damagefrom arcing and sparks caused thereby.

More than 200 crystal structures and forms of silicon carbide have beenidentified. Some forms of silicon carbide are known to heat uponabsorption of microwave energy, and can be used in various forms inmanufacture of objects that heat by absorption of electromagneticradiation. Silicon carbide thus used is polycrystalline particulate.Silicon carbide is used to make objects that are heated by absorption ofmicrowave energy. Such objects are used, for example, as reversiblewater absorbents, i.e., objects that absorb water that then are driedand rejuvenated by heating in a microwave oven to drive off the absorbedwater. Other forms of silicon carbide are known to be minimallyabsorptive of microwave radiation.

Absorptive forms of silicon carbide can be used as a part of productsused for heating foods. In particular, silicon carbide is used formanufacture of heating objects used to form a browned surface on foodsheated in a microwave oven, because microwave energy alone often doesnot brown foods. Also, silicon carbide whiskers are described as mineralfiller, providing rigidity and strength, for resins used to makecontainers used in microwave cooking. When used as a mineral filler,silicon carbide usually is in a form that is minimally absorptive ofmicrowave radiation.

Silicon carbide also is used in selected steps of processes formanufacture of ceramic and metallic objects. For example, organic binderhas been removed from an object made from silicon carbide powder byheating in microwave energy. Also, silicon carbide powder is known as amicrowave-absorbent material suitable for heating ceramic pellets buriedtherein to degrease the pellets, or to remove binder and sinter thepellets. Graphite, silicon carbide, and other di-electric materials areknown to be suitable material to be embedded in a polymeric ceramicprecursor system containing a metal element that is, upon exposure tohigh-frequency (greater than 20 GHz) microwave energy, cured by heatproduced by the di-electric to form a ceramic/metal composite. However,these systems typically have slow heating rates, thereby negating thebenefits associated with microwave heating.

It is known that exposing silicon carbide, ceramic fibers, and microwaveabsorptive materials to microwave energy may lead to undesirable arcingand sparking. Metals, including metal powders, also lead to arcingbecause metals are conductive. Therefore, methods of reducing suchsparking have been developed. In one such method, silicon carbide isdeposited in and around the ceramic fibers by chemical vapor deposition.Such ceramic/silicon carbide composites heat when exposed to microwaveenergy, but the silicon carbide formed by CVD suppresses sparking.

Silicon carbide and other carbon-containing materials are mixed into aceramic-containing powder mixture to serve as an aid to the heating of aceramic powder thick-walled object during sintering. The powder containsmaterials, such as clays and kaolins, to increase susceptibility tomicrowave energy, and the object is autoclaved before microwaving to putthe ceramic powders into a form that absorbs microwave energy moreefficiently than the unchanged ceramic. The object then is sintered byexposure to microwave energy. However, if the object is not autoclaved,the heating rate is unacceptably slow.

There exist limitations on the use of silicon carbide in heat-generatingobjects. The heating efficiency of even the most absorptive forms ofsilicon carbide typically is low. Indeed, the heating efficiency oftypical silicon carbide particulate is so low that such particulate isused to increase the strength and cut-resistance of microwave-heatablefood containers that remain cool to the touch. It also has beendisclosed that addition of silicon carbide to plastics of various typesyields a product that imparts objectionable odors during use. Thus, useof such silicon carbide forms requires ameliorating measures andadditives to control these odors.

Typically, the highest temperature achieved by irradiation of siliconcarbide with microwave energy is relatively low, about 300° C., oftenbecause the concentration of silicon carbide must be kept low topreclude degrading the properties and characteristics of the matrixmaterial. Higher temperatures can be achieved in combination with somematerials such as solid blocks comprising silicon carbide bonded withanother material. Temperatures of 600-800° C. are achieved within 3-4minutes of exposure to microwave energy at 600 W with solid blocks ofsilicon nitride-bonded silicon carbide. Such material has a densitylower than solid or single crystal silicon carbide. Such material isused for ashing other compounds.

There remains a need for improved radiant heating devices. Devices forfood heating are limited, as are devices for other purposes. Knowndevices are inefficient and slow to achieve working temperatures. Theslow heating rates negate the benefits, and especially the expectedquick heating, of microwave and radiant heating methods. Some devicesare limited by the temperatures that can be achieved. Others causearcing when exposed to microwave energy, thus risking damage to themicrowave oven or electromagnetic radiation source. The quantity ofsilicon carbide powder, carbon powder and particles, and othermicrowave-absorbing material required to achieve a desired heatgeneration often must be so great as to degrade the properties andcharacteristics of the matrix to which the material is added. Thus,there remains a need for devices that heat quickly, efficiently, and toa high temperature, without arcing and sparking, when exposed toelectromagnetic radiation, especially microwave energy. Also, thereexists a need for objects that have a wider range of uses than now areavailable.

BRIEF SUMMARY OF THE INVENTION

The invention is directed to composite materials that heat quickly andefficiently to a high temperature by absorption of electromagneticradiation, and particularly by absorption of microwave energy. Inaddition, composite materials of the invention do not arc or causesparking. The composite materials comprise a unique form of siliconcarbide. This unique form is single-crystal silicon carbide whisker orfibril that is strongly heated by electromagnetic radiation. Theinvention also is directed to objects made with the composite materials,and to a method of heating using the materials of the invention.

Composite materials of the invention can be shaped to form containers,such as crucibles, cooking, and serving vessels that are suitable forheating other compositions contained therein or thereon by absorbingelectromagnetic radiation. Composite materials of the invention also canbe formed into shapes and used as a source of heat, such as for meltingmetals. Materials of the invention also can be microwave-curable ormicrowave-heatable reinforced polymers and adhesives.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a comparison of microwave heating of mineral oil-basedmaterials.

FIG. 2 illustrates the relationship between the temperature reached by acomposite material of the invention and the concentration of singlecrystal silicon carbide whiskers in the composite material.

FIG. 3 illustrates a comparison of microwave heating of alumina-basedmaterials.

FIG. 4 illustrates the dielectric loss tangent as a function of thesingle crystal silicon carbide whiskers in an alumina matrix.

FIG. 5 illustrates a comparison of the heating ability ofcommercially-available silicon carbide as compared with single crystalsilicon carbide whiskers.

FIG. 6 depicts the average heating rate for composite material of theinvention comprising single crystal silicon carbide whiskers withalumina.

FIG. 7 depicts the heating ability of composite material of theinvention compared to comparative samples prepared with polycrystallinesilicon carbide particles.

FIG. 8 depicts the average heating rate for composite material of theinvention in the form of a sintered, fused ceramic rod.

FIG. 9 depicts the heating rate of aluminum powder that was melted andfused into a solid mass by heating with composite of the invention.

FIG. 10A is an isometric depiction of a heating rack according to oneembodiment of the invention. FIGS. 10B and 10C are cross-sections ofheating element 1 of FIG. 10A.

FIG. 11A is an isometric depiction of another heating rack according tothe invention. FIGS. 11B and 11C are alternate cross-sections of heatingelement 1 of FIG. 1A.

FIG. 12A is an isometric depiction of another heating rack according tothe invention. FIGS. 12B, 12C and 12D are alternate cross-sections ofheating element 1 of FIG. 12A.

FIG. 13A is a depiction of a microwave heatable tray 10 according to theinvention having a rectangle configuration and discrete microwaveheatable elements 11.

FIG. 13B is an alternative circular configuration for tray 10.

DETAILED DESCRIPTION OF THE INVENTION

The invention is directed to composite materials that heat quickly andefficiently to a high temperature by absorption of electromagneticradiation, particularly of microwave energy. The inventors havesynthesized single crystal silicon carbide whiskers and fibrils and havediscovered that these single crystal silicon carbide whiskers andfibrils are especially suited to converting electromagnetic radiation,particularly microwave energy, to heat. Thus, objects made with thecomposite materials heat quickly and efficiently, and to a hightemperature if desired. Thus, such objects can be used in manners notsuitable for known silicon carbide-containing microwave-heatabledevices, such as to melt metal. In the alternative, objects that do notrequire higher temperatures can be made at lower cost and used at lowercost. Further, single crystal silicon carbide whiskers and fibrilsprovide additional strength as compared with objects containingparticulate silicon carbide, and objects containing the single crystalsilicon carbide whiskers or fibrils are more easily manufactured thanare objects made from known materials. The quantity of single crystalsilicon carbide whiskers or fibrils is sufficiently low so as not todegrade the properties and characteristics of the matrix material.

Composite materials of the invention comprise single crystal siliconcarbide whiskers or fibrils in a matrix or binder. The skilledpractitioner recognizes that conversion of electromagnetic radiation toheat within an absorptive material is governed by the followingequation:P=2πfE ²∈ tan δ  [1]

where:

-   -   P=power (watts/m³)    -   f=frequency (Hz)    -   E=voltage gradient (V/m)=    -   ∈=dielectric permittivity    -   tan δ=dielectric loss tangent

The inventors have discovered that, surprisingly, when compositematerials are formed with the single crystal silicon carbide whiskers orfibrils of the invention, the dielectric loss tangent value of thecomposite rises to very high levels. This effect results in thesecomposites having exceptionally rapid heating rates.

A composition having a high loss tangent value is said to be a ‘lossy’composition. That is, such a composition generates a large loss(dissipating) current, and so heats well. Compositions with lowdielectric loss do not heat well when exposed to electromagneticradiation. Thus, most ceramics, polymers, inks, adhesives, dry wood andcellulosic products (including paper and cardboard), and mineralsgenerally exhibit low dielectric loss (i.e., are not lossy) andtherefore do not convert electromagnetic radiation efficiently intoheat.

The inventors have discovered that single crystal silicon carbidewhiskers and fibrils have a remarkable ability to absorb and convertelectromagnetic radiation into heat. Therefore, single crystal siliconcarbide whiskers and fibrils can be used at lower loading levels, i.e.,at lower concentration, compared with polycrystalline silicon carbideparticles, to achieve a desired level of heat generation. In particular,the ability to use a lesser amount of single crystal silicon carbidewhiskers or fibrils than polycrystalline silicon carbide particlesattains the advantage that the properties and characteristics of thematrix material are not degraded. Further, single crystal siliconcarbide whiskers and fibrils do not arc or cause sparks.

Although the inventors do not wish to be bound by theory, it is believedthat single crystal silicon carbide whiskers and fibrils have aparticularly high dielectric loss tangent because they have favorablemorphology. Also, dopants in single crystal silicon carbide whiskers andfibrils provide additional heat release per unit energy absorbed.Dopants operate by increasing the value of the dielectric loss tangent.Dopants can be placed inside the silicon carbide lattice itself, orwithin the composite matrix surrounding the silicon carbide.

There are two types of dopants: “p-” type and “n-” type. A p-type dopantincreases the number of free (in this case positive) charge carriers.When a p-type doping material is added, it takes away (accepts)weakly-bound electrons in the silicon carbide lattice. This creates alost electron “hole.” Having more holes increases the conductivity ofthe material, which in turn increases dielectric loss tangent value.

Conversely, an n-type dopant gives away (donates) weakly-bound electronsto the silicon carbide lattice. This type of doping agent is also knownas donor material since it gives away some of its electrons. The purposeof n-type doping is to produce an abundance of mobile or “carrier”electrons in the material. A typical silicon carbide structure iscovalently bonded with four electrons. An n-type dopant introduces afifth electron into the structure. This fifth electron is mobile, andthe electron mobility increases the conductivity, which in turnincreases the dielectric loss tangent value.

The inventors have found that n-type dopants are particularly useful forgenerating higher dielectric loss tangent values. In particular,dissolved nitrogen is a very effective n-type dopant to increase thedielectric loss tangent of the silicon carbide lattice. Theconcentration of dopant in single crystal silicon carbide whiskers andfibrils used to form composite material of the invention ranges up toabout 2 wt percent, preferably up to about 1.5 wt percent, and morepreferably up to about 1.0 wt percent. Any amount of dopant willincrease the lossiness of the single crystal silicon carbide whiskersand fibrils. For example, nitrogen present at a concentration of betweenabout 0.2 and about 0.5 wt percent increases the lossiness of singlecrystal silicon carbide whiskers and fibrils. With the guidance providedherein, the skilled practitioner will be able to identify suitabledopants and determine concentrations of dopants that provide a desiredincrease in lossiness.

Single crystal silicon carbide used in the invention comprises twoform-factors, whiskers and fibrils. Whiskers and fibrils can be usedseparately or in combination of any proportions. The whiskers aregenerally smaller than the fibrils, but the size ranges may overlap. Thewhiskers have a diameter of between about 0.2 and about 10 microns,preferably between about 0.3 and about 3 microns, more preferablybetween about 0.4 and about 2 microns, and most preferably between about0.5 and about 1.5 microns. The aspect ratio, i.e., the ratio of lengthto diameter (L/D), of such whiskers is between about 10 and about 100,preferably between about 10 and 50, and more preferably between about 12and about 20. One such commercially available single crystal siliconcarbide whisker product is available from Advanced Composite MaterialsCorporation of Greer, S.C., under the trade name Silar® brand siliconcarbide whiskers. This product comprises single crystal silicon carbidewhiskers having an average diameter of 0.6 microns and an average lengthof 9 microns. Single crystal silicon carbide whiskers can be made inaccordance with the method disclosed in Cutler, U.S. Pat. No. 3,754,076,the entirety of which is hereby incorporated by reference. Siliconcarbide fibrils suitably used in this invention typically have adiameter of about 4 to about 20 microns, preferably between about 4 andabout 16 microns, and more preferably between about 5 and about 15microns. These fibrils have a length at least about 50 microns, andtypically between about 60 and about 1000 microns. Such fibrils can bemade in accordance with the methods disclosed in Angier, United StatesPatent Application Publication 2004/0009112, and Angier, U.S.application Ser. No. 11/186,941, the entireties of which are herebyincorporated by reference.

Thus, the morphology of the single crystal silicon carbide whiskers andfibrils affects the dielectric loss tangent value. The morphology of thesingle crystal silicon carbide whiskers and fibrils is particularlywell-suited to provide radiant heating when exposed to electromagneticradiation, and is superior to the morphology of bulk or granular formsof polycrystalline silicon carbide for this purpose.

Silicon carbide whiskers are single crystal materials. Because they arenot polycrystalline, there are essentially no voids, defects, ordisruptions in the crystal lattice that can limit conductivity andthereby reduce the ability of the whiskers to heat.

Silicon carbide fibrils may be, but typically are not, single crystals.Rather, these fibrils are likely to be made up of numerous singlecrystals of silicon carbide. Although the inventors do not wish to bebound by theory, silicon carbide fibrils are believed to benefit fromthe same effect as single crystal silicon carbide whiskers, becausethere are believed to be single-crystal segments within these fibrilsthat are advantageously conductive. Therefore, unlike polycrystallinesilicon carbide of comparable dimension, there are few crystalboundaries that may lead to voids, defects, and disruptions in thecrystal lattice.

Although the inventors do not wish to be bound by theory, siliconcarbide whiskers with a diameter of 0.5 microns and an aspect ratio ofat least 10:1 (length greater than 5 microns) allow for electron flowover a larger distance than polycrystalline silicon carbideparticulates. This electron flow over a longer span creates line loss,and increases the dielectric loss tangent. Likewise, silicon carbidefibrils, which more preferably have a diameter from 5-15 microns andlength of at least 50 microns, can have large line loss. As anelectrical potential is applied to one side of the whisker or fibril, adopant's donor electron will flow. Since the whisker or fibril is oracts like a long single crystal, the free electron flow lengthincreases, which in turn increases the line loss and the dielectric losstangent value.

Thus, single crystal silicon carbide whiskers and fibrils comprises (1)single crystal whiskers and (2) fibrils having numerous single crystalsegments. Although fibrils may be slightly less efficient at conversionof microwave energy to heat than single crystal whiskers, fibrils havean efficiency essentially the same as the single crystal silicon carbidewhiskers, and have properties and characteristics much more like singlecrystal silicon carbide whiskers than typical polycrystalline siliconcarbide.

The matrix material is selected to provide a composite material of theinvention suitable for the intended use. For example, a compositematerial of the invention used to melt metal will typically utilize amatrix material different from a matrix material used to form acomposite material of the invention used to heat food.

Matrix material for composite materials of the invention should resistdeformation caused by the increase in temperature expected when thecomposite object is heated. Absorption of microwave energy by the matrixmaterial may reduce the quantity of microwave energy available to thesingle crystal silicon carbide whiskers and fibrils. Preferably,therefore, the matrix material is transparent to microwave energy so asto make essentially all of the microwave energy available to heat thesingle crystal silicon carbide whiskers and fibrils.

Typically, matrix material suitably used in composite material of theinvention preferably has a high dielectric permittivity to theelectromagnetic radiation used, and is selected from polymers, ceramics,organic liquids and solids, and any other composition that istransparent to or heat only slowly upon exposure to electromagneticradiation. The electromagnetic radiation can have a frequency betweenabout 5 kHz (radiowave frequencies) and about 150 GHz (microwavefrequencies). More typically, however microwave frequencies in the rangeof from about 500 MHz to about 23 GHz, and more typically between about750 MHz and about 3 GHz. Typically, microwave ovens use a frequency ofabout 915 MHz (industrial ovens in the United States) and about 2.45 GHz(consumer ovens in the United States). Other frequencies may be used aswell, such as 896 MHz, which is less common.

Preferably, composite objects of the invention do not melt, warp, orotherwise become objectionably deformed when heated. For example, a foodservice container preferably retains its shape well, as distortions,bulging, bending, melting, and similar deformations likely would beperceived as objectionable by consumers. However, users likely would notobject to such minor deformations in an object intended for industrialuse as a heat source. Also, the matrix material must be suitable for theuse intended. For example, food service equipment must be manufacturedwith food-safe material. With the guidance provided herein, the skilledpractitioner will be able to identify and select suitable matrixmaterial.

Thus, the matrix material typically is selected frommicrowave-transparent compounds, and is selected based on the propertiesand characteristics required for the use intended. Matrix materialtypically is selected from organic and inorganic solids and liquids,ceramics, and polymers. Polymers can be thermoplastic or thermoset,although there exists the issue of melting a thermoplastic polymer.Also, hot-melt adhesives and polymers that cure with heat also aresuitable matrix material for single crystal silicon carbide whiskers andfibrils to form composite materials of the invention.

In addition, thermosets can be ‘activated’ above their glass transitiontemperatures by way of microwave heating. When activated above its glasstransition temperature, a thermoset changes from a hard, glassy polymerinto a soft, rubbery elastomer. Therefore, microwave ‘activation’ can beused to raise the temperature of a thermoset polymer and thereby movethe polymer from the glassy state through the glass transitiontemperature and into the rubbery state.

For polymers in general and in food service, for example, matrixmaterial includes acrylics, polyetherimide, polyamide, polyphenyleneether, aliphatic polyketone, polyetherether ketones, polysulfones,aromatic polyesters, silicone resins, epoxy resins, polyolefins, such asisotactic polypropylene, polypropylene- and polyethylene-basedcopolymers, and, for particularly high temperature food-related uses,polyphenylenesulfides. Blends of compatable polymers and polymerprecursors cured in place also are suitable for use in food service.

Fluids and pastes also are suitable matrix material. Mineral oil(paraffin oil) is suitable as a matrix material. Mineral oil with singlecrystal silicon carbide whiskers and fibrils added thereto and placedinto a suitable flexible container results in a mass that comports toanother shape. Thus, this configuration forms a heating pad convenientfor use on the human body, for example. Other fluids, such as oils,alcohols, and other organic liquids, also can serve as matrix material.

Other uses call for a durable composite material of the invention. Suchmaterials include, for example, compounds and mixtures of alumina,zirconia, silica, silicon and boron nitrides, titanium compounds, andother ceramics. Similarly, ceramic papers, such as filter papers, can bemade with single crystal silicon carbide whiskers and fibrils inaccordance with this invention. Other suitable matrix materials includezeolites and molecular sieves.

Other matrix materials include silicone rubber and othermicrowave-transparent rubbers. Films formed of polyolefin, polyester,polyimide, fluorocarbon, silicone, nylon, and polyether sulfone can beused to affix a coating comprising single crystal silicon carbidewhiskers and fibrils to paper, cardboard, and the like. Single crystalsilicon carbide whiskers and fibrils in a carrier serves as a compositematerial of the invention that can be applied to a substrate to form acoating of single crystal silicon carbide whiskers and fibrils in aretentive base.

Composite materials of the invention also can be in the form of acoating or paint comprising single crystal silicon carbide whiskers andfibrils that sets or forms a desired coating on a substrate. Forexample, single crystals of silicon carbide can be dispersed withpolyphenylene sulfide (PPS) in pentachloronitrobenzene orp-dichlorobenzene at elevated pressure and temperature above about 200°C. This dispersion can be coated onto glass or anothermicrowave-transparent substrate to make an object with a microwaveactive coating.

Composite materials of the invention include hot-melt adhesivescomprising microwave absorbent single crystal silicon carbide whiskersand fibrils. Exposure of such an adhesive type, which typically existsin the form of a solid material that is transparent to microwave energy,to microwave energy would cause the adhesive to activate or to melt. Anexample of one such adhesive is a combination of single crystal siliconcarbide whiskers and fibrils in an adhesive comprising a minor amount(about 20 wt percent, based on the weight of the organic materials) ofparaffin wax, with the remainder of the organic adhesive comprisingethylene-vinyl acetate.

Polymeric materials comprising precursors that cure by cross-linkingwith heat also are suitable matrix materials for composite materials ofthe invention. These materials can be inorganic, organic, or hybrid.Such materials include polycarbosilanes (to form a silicon carbidematrix). Other suitable materials are selected from mixtures of polymercerams. Sol-gels that form ceramic powders also may serve as a matrixmaterial.

Thus, composite materials of the invention have many forms. For example,a liquid or paste matrix material likely yields a liquid or pastecomposite material of the invention. Similarly, a powder matrix materialyields a powder composite material of the invention. However, suchpowders also may be pressed or cast into larger solid forms, such asrods, bricks, engineered parts, and the like. Resins, whether thermosetor thermoform, may be mixed with single crystal silicon carbide whiskersand fibrils, then shaped by various polymer processing methods and setor formed to yield composite material of the invention having thedesired shape. Flexible composite materials of the invention may beformed from silicone or organic rubbers comprising single crystalsilicon carbide whiskers and fibrils.

Similarly, objects comprising composite material of the invention aremade in accordance with known methods as appropriate for the physicalform of the matrix material. For example, single crystal silicon carbidewhiskers and fibrils need only be mixed into a fluid, a paste, or amolten resin. Solid particulate matrix material can be mixed with orblended with single crystal silicon carbide whiskers and fibrils. Suchparticulate composite material of the invention then may be formed intolarger solids, such as bricks, pills, rods, spheres, or any desiredshape and size, by casting, pressing, or otherwise agglomerating theparticulate product.

With the guidance provided herein, the skilled practitioner will be ableto select a matrix material suitable for the use intended into whichsingle crystal silicon carbide whiskers, fibrils, and blends thereof,can be introduced to form a composite material of the invention.

The proportion of single crystal silicon carbide whiskers and fibrils inthe matrix material is a function of the properties and characteristicsof the matrix material and the degree of heat generation desired. Forexample, the quantity of single crystal silicon carbide whiskers,fibrils, or blend thereof, in a thermoplastic polymer resin, especiallyone having a relatively low melting point, will be relatively lowbecause care must be taken not to melt the plastic. Similarly, care mustbe taken to avoid setting fire to wood and cellulosic products, such aspaper and cardboard. In contradistinction, ceramics are resistant tohigh temperatures and may be used in heat generation at hightemperature. Thus, the concentration of single crystal silicon carbidewhiskers, fibrils, or a blend thereof, in such a composition of theinvention can be relatively high.

Thus, the lower limit of single crystal silicon carbide whiskers andfibrils in the matrix material is that level that yields the desiredheat generation. Similarly, the upper limit of single crystal siliconcarbide whiskers and fibrils in the matrix material is that quantitythat adversely affects the properties and characteristics of the matrixmaterial. However, this limit is higher for single crystal siliconcarbide whiskers and fibrils than for known silicon carbideparticulates. Known silicon carbide particulates tend to degrade theproperties and characteristics of the matrix material as theconcentration increases, particularly because they cause the matrixmaterial to lose cohesive strength at high concentrations. Also, knownsilicon carbide particulates do not ameliorate thermal shock well.However, single crystal silicon carbide whiskers and fibrils tend tostrengthen the material because the single crystal silicon carbidewhiskers and fibrils are rigid rods. Further, this morphology rendersthe composite material of the invention highly resistant to thermalshock.

Typically, therefore, the concentration of single crystal siliconcarbide whiskers and fibrils in the composite material of the inventionis between about 0.1 and about 95 wt percent, based on the total weightof the single crystal silicon carbide whiskers and fibrils and matrixmaterial. More typically, the concentration of single crystal siliconcarbide whiskers and fibrils in the composite material of the inventionis between about 1 and about 90 wt percent on the same basis, and evenmore typically is between about 5 and about 75 wt percent on the samebasis. The skilled practitioner can, with the guidance provided herein,calculate or otherwise determine an appropriate concentration of singlecrystal silicon carbide whiskers and fibrils in a matrix material thatwill yield the desired heat generation in the desired application.

The skilled practitioner recognizes that various factors will affect theheating rate upon exposure of composite material of the invention. Ascan be seen from equation 1 above, heating rate will increase with ahigher wattage. Heating rate also increases as electromagnetic radiationfrequency increases. If the article to be heated is insulated, higherheating rates will result, due primarily to reduced heat transfer to thesurroundings.

Both the uniformity of the energy field and the location of the objectto be heated within that field also affect the heating rate. As theskilled practitioner recognizes, electromagnetic radiation fields canhave irregularities in them. In particular, due to the long wavelength(10-12 cm), microwave fields have ‘hot spots’ and ‘cold spots’ that canyield different heating rates.

Both convective forces and conductive forces also affect the heatingrate. Forced convection, with a fluid or gas flowing over the element tobe heated, will result in lower heating rate, because forced convectionlowers the temperature in the sample by transferring heat to thesurrounding fluid. Conductive forces also serve to reduce heat flow whenthe composite material is attached to a material, such as metal, thathas high thermal conductivity, as the heat readily flows through suchmaterials, such as a metal. Thus, although there are many factors thatcan affect heating rate, composite material of the invention yields amarked improvement over known techniques.

Composite material of the invention has many uses. One suitable consumeruse is in food processing articles. Food processing articles include,for example, bakeware, cookware, ovenable containers, mixing bowls,measuring cups, blender bowls, serving items such as plates, platters,browning platters and containers, cups, and items of this type, foodheating and browning devices, and the like. An insulated vessel, asdescribed herein, also may be used as a food processing article. Suchfood processing articles comprising composite material of the inventioncan be used to cook, bake, or otherwise process food by heating byexposing the article, or the article with food therein or thereon, toradiowave or microwave energy. Such articles include, inter alia, thosedescribed in U.S. Pat. Nos. 4,931,608; 6,608,292; 6,229,131; 7,154,073;7,176,426; US Publication 2003/0218010; WO90/03716; WO00/35251;WO2006/068402 and EP 1,781,070.

Vessels comprising composite material of the invention are suitably usedfor heating or warming any object or item. For example, such a vessel issuitable for heating linseed oil or other coatings that preferably areapplied warm or are heated before application. Similarly, such a vesselcould be used to melt thermoplastic resins. The skilled practitionerrecognizes that such vessels have a myriad of uses and, with theguidance provided herein, can make and use such vessels for any purpose.

Similarly, insulated and uninsulated vessels for heating a variety ofitems also can include composite material of the invention. Such vesselsmay be suitable for food use, or may be used for heating, and keepingwarm, any object. For example, an insulated cup comprising compositematerial of the invention for coffee, tea, hot chocolate, or otherbeverages, or an insulated vessel comprising composite material of theinvention, such as a bowl for foods, would both heat food and keep itwarm thereafter. Also, an insulated vessel comprising composite materialof the invention is useful for heating materials that require relativelyhigh temperature, such as for melting or fusing metals. One such vesselis described in Example 9.

In one embodiment of the invention there is provided a rack suitable foruse in an electromagnetically heated oven comprising support memberssupporting discrete, spaced apart longitudinal radiation absorbingelements comprised of single crystal silicon carbide whiskers, fibrilsor a blend thereof in a matrix material, said radiation absorbingelements being supported so as to provide space between the radiationabsorbing elements and the surface upon which the rack rests.

This embodiment of the invention may be better understood with referenceto FIGS. 10A, 11A and 12A, exemplifications of the embodiment.Longitudinal radiation absorbing elements 1 are supported by channels 2in a spaced-apart relationship. The channels 2, in turn, are supportedfrom base 3 to provide space between the radiation absorbing elementsand the surface upon which the rack rests. In addition, this rack andits longitudinal elements can be advantageously placed in the oven,typically above or below the food to be cooked, to achieve desiredcooking effects.

The longitudinal radiation absorbing elements 1 can have a cross-sectionthat is curvilinear, circular, (See e.g., FIG. 10B) polygonal havingthree or more sides (See e.g., FIG. 11B) or any custom geometries suchas a t-rail design. (See e.g., FIG. 12B-D). Elements having a circularcross-section may have a diameter, for example, from about one quarterinch to 1 inch or more, having due regard for the strength needed tomaintain the structural integrity of the element when in use. Elementshaving other cross-sections can be of comparable size. In onealternative, the elements may be hollow as shown, for example, in FIGS.10C and 11C.

The silicon carbide and matrix materials from which the longitudinalelements may be made are those described above. A preferred combinationis silicon carbide whiskers and/or fibrils in an alumina matrix. Thealumina matrix may include sintering aids and toughening components suchas disclosed, inter alia, in Rhodes et al., U.S. Pat. No. 4,961,757 andRogers et al. U.S. Pat. No. 5,389,586. One typical suitable formulationcontains 90.5% aluminum oxide, 0.9% yttrium oxide, 0.9% magnesium oxideand 7.7% silicon carbide whiskers. The choice of an alumina matrixprovides elements which maintain their physical and mechanical integrityat operating temperatures as high as 600-800° F. or higher. That issufficiently high to permit self-cleaning temperatures to be achieved ifdesired. Moreover, the finished alumina matrix elements can have aporosity of less than about 2%. They are hydrophobic, and at such lowporosity, they do not absorb liquid from the food.

The longitudinal elements can be prepared employing technologiesanalogous to those known in the art as disclosed, inter alia, in Rhodeset al., U.S. Pat. No. 4,961,757, Rogers et al., U.S. Pat. No. 5,389,586and Dugan et al., U.S. Pat. No. 5,398,858. For example, the siliconcarbide whiskers and/or fibrils can be thoroughly dispersed throughoutthe matrix and the blend can be shaped and presintered by conventionalmethods. Elements with the alumina matrix can be axially pressed at, forexample, 15-25 ksi, or extruded to produce a green product having adensity of 50-75% theoretical. The green product can be sintered toproduce a product having 95-100% theoretical density. Sinteringtemperatures of 1700° C. to 1850° C. can be employed. Extrusion providesa product in which the whiskers and/or fibrils tend to be alignedthereby adding to the toughness and fracture resistance of the product.

The longitudinal elements may be supported by any of the variety ofmeans including channels 2 at the ends of the elements, although thesupports may be located at positions other than the ends of theelements. Each element may also be individually supported. The choice ofphysical support means to maintain the elements in spaced-relationshipabove the base 3 of the rack is easily within skill of the art.

The spacing of the elements is also a matter of design choice so long asthe space between elements is sufficient to, if required, allow anyfluids present in food being processed to pass downwardly from the uppersurface of the longitudinal element and provide sufficient support forfood being processed. Because the composite material of the inventiondoes not arc, the longitudinal elements can be abutted next to eachother, allowing for maximum heat of the food via conduction (grilling),convection (in conjunction with air flow), or through infraredradiation. In addition, if desired, such spacing may, for example, beabout one-eighth of an inch to about one inch or more.

The rack includes a base element 3 as shown in FIGS. 10A, 11A and 12A,which may be made of steel, ceramic, resin or the like. The function ofthe base is to maintain the longitudinal elements in a spacedrelationship above the base of the oven. While the base is shown to berectangular, it can also have other configurations such as circular forrotation within an oven. If desired, the space between the elements andthe base of the oven, coupled with the space between elements, can bedesigned to allow for air flow and convection heating. Alternatively,the elements can be abutted together to provide for maximum heating viagrilling and conduction. The base may also include provision for a panmember to collect fluid which exits the food during cooking. Once againthe choice for a pan is easily within the skill of the art. For someapplications, it may be desirable that the pan not be made ofelectromagnetic heat-absorbing material since its function is not tocook but to collect fluids. In other applications, it may be desirablethat the pan be made of electromagnetic heat-absorbing material.

In another embodiment of the invention, as shown in FIGS. 13A and 13B, amicrowave heatable tray 10 is used. A housing tray 10 is formed whichholds and spaces the composite material of the invention in discreteelements 11. Preferably, this housing tray is metallic, although it canbe formed of high temperature polymeric materials which are commonlyused in food applications, e.g., poly (phelyene sulfide). In addition,the tray can be ceramic itself. As mentioned, a metal housing tray ispreferred because it is durable and will allow for rapid heat transferfrom the ceramic composite into the entire tray. This housing tray canbe stamped, to enable the discrete composite ceramic to be placed in therecesses of the metal. The ceramic can be connected to the metal viabrazing, high temperature adhesives, or through mechanical means. Foodcan be placed on the tray and can be actively cooked or baked. Varioustray geometries are envisioned, including circular, square, andrectangular trays. In addition, various discrete ceramic elementgeometries 11 are also possible, including small tiles, rectangles andcircles. The tray can be inserted or removed from the oven as desired.

It is preferred that the microwave absorbent ceramics in the tray ofFIGS. 13A and B be discrete. If the ceramics on this tray arecontinuous, or too large, or if the entire tray is made up entirely ofthe composite ceramic of the invention, the tray itself can be subjectedto thermal stresses that will shorten the usable life of the tray.

Composite material of the invention also is suitably used in manufactureof drying apparatus. One type of drying apparatus is used for absorbingwater or another vapor or gas, particularly from the atmosphere, thendesorbing that water, vapor, or gas by heating. Such drying apparatussuitable is made with a desiccant, which the skilled practitioner canselect, and with composite material of the invention. The dryingapparatus is used by heating the apparatus to desorb water, vapor, orgas, then allowed to cool. The cooled apparatus then is used to absorbmaterial, which then is desorbed by heating, upon exposure to radiowavesor microwaves.

Another type of drying apparatus involves combination drying by directmicrowave and the composite material of the invention. Because microwaveirradiation is known to have poor field uniformity, microwave drying canbe problematic. Due to non-uniform field distribution, the material tobe dried may end up being well heated and dried in one area, butunder-heated with undesirably high moisture content in another area,after the microwave field is applied. However, a combination dryercomprising the composite material of the invention will therefore haveboth direct microwave heating of the water to be driven off, in additionto convective, conductive, or infrared heating induced by the compositematerial of the invention. Advantageously, this combination heating anddrying is performed inside a single microwave drying apparatus. Theresult is more uniform, complete, and efficient microwave drying thanwould be experienced by a microwave field alone.

Composite material of the invention also can be used in the manufactureof a combination oven. Composite material of the invention is suitablyused to form the convection heating element of a combinationmicrowave-convection oven. Such an oven thus need have only a magnetron,and not a magnetron plus an electric heating element. Rather, themicrowaves are used to heat the object in the oven directly, and themicrowaves also heat the composite material of the invention. The heatedcomposite material of the invention thus serves as the convectiveheating element.

With the guidance provided herein, the skilled practitioner will be ableto manufacture and use such food processing articles, insulated anduninsulated vessels, drying apparatus, combination ovens, and otheritems.

In accordance with the method of the invention, an object comprising acomposite material of the invention, or a composite material of theinvention itself, is heated by exposure to electromagnetic radiation,typically microwave energy. Examples 1-5 illustrate such heating. Anobject to be heated is placed in contact with or in close proximity withthe composite material of the invention while the composite material ofthe invention generates internal energy in the form of heat. An exampleof this method is the use of the heated composite material of theinvention as a heat source out of the irradiation zone, such as aheating pad or trivet for keeping food warm after the food has beencooked and before it is consumed. Heat is transmitted by, e.g.,conduction, convection, or re-transmittal by infrared irradiation to theobject.

Also in accordance with the method of the invention, the object to beheated can be placed in the radiation zone while the composite materialof the invention is being heated. Examples 6-9 illustrate this method.Another example of this method is heating food in a microwave oven on aplate comprising composite material of the invention to heat and/orbrown the food. With the guidance provided herein, the skilledpractitioner will be able to practice the method of the invention.

EXAMPLES

The following examples are intended to illustrate the invention, and notto limit it in any way. For example, electromagnetic radiation of adifferent wavelength or power can be utilized in accordance with theinvention. Similarly, the matrix materials exemplified herein are notthe only matrix materials that can be used in accordance with theinvention. With the guidance provided herein, the skilled practitionerwill be able to practice the invention by making composite materials ofthe invention and use these materials in accordance with the method ofthe invention to heat objects quickly and efficiently, and to atemperature higher than those typically achieved by exposure tomicrowave energy if desired, by exposing the composite material toelectromagnetic radiation, particularly to microwave energy.

In Examples 1-5, single crystal silicon carbide whiskers having anaverage diameter of 0.6 microns and an average length of 9 microns wereused. The polycrystalline silicon carbide particulate used in comparisonexamples was particulate silicon carbide having an average size of lessthan 1 micron. The wattage value stated in an example is the nominalwattage of the magnetron used.

Example 1 Heating of a Low-Loss Dielectric Fluid

Refined mineral oil of 100% hydrocarbon (paraffin oil) content has a lowdielectric loss when subjected to electromagnetic radiation, includingmicrowave radiation. Therefore, it does not heat when exposed tomicrowave radiation. Single crystal silicon carbide whiskers (0.6 micronaverage diameter, 9 micron length) and ultrafine silicon carbideparticulate (<1 micron average size) were added to separate aliquots ofmineral oil at 1% by weight, and separately subjected to a microwavefield of 2.45 GHz at 100 watts. The results are shown in Table 1 andillustrated in FIG. 1.

TABLE 1 Temperature Increase, ° C. Mineral Oil (1% Mineral Oil (1% Time,Single Crystal Silicon Carbide Pure Mineral seconds Silicon Carbide)Particulate) Oil 5 3.0 0 0 10 8.0 1.0 0 20 19.7 2.3 0 30 32.3 4.7 2.7

As expected, pure mineral oil exhibits only a 2.7° C. temperature rise.Mineral oil with 1% silicon carbide particulate exhibits 4.7° C.temperature rise. Surprisingly, mineral oil with 1% by weight of singlecrystal silicon carbide whiskers exhibits a 32.3° C. temperature rise.The single crystal silicon carbide whiskers used in the invention werefar superior to fine powder forms of silicon carbide in powerdissipation, and therefore in resultant temperature increase.

Example 2 Heating of a Low-Loss Dielectric Fluid

Single crystal silicon carbide whiskers used in the invention were againadded to 100% paraffin mineral oil. In this example, the silicon carbideof the invention was added at increasing weight percentages, with thegoal of increasing the dielectric loss tangent with increasing weightpercent single crystal silicon carbide whiskers. Each sample wasseparately subjected to a microwave-energy field of 2.45 GHz at 1000watts for 20 seconds. The temperature rise is shown in Table 2 below,and is illustrated in FIG. 2:

TABLE 2 Wt percent Single Crystal Silicon Carbide Whiskers In MineralOil Temperature Increase, ° C. 0 0 0.5 8.7 1.0 19.7 2.0 32.0 4.8 68.79.1 122.3

As can be clearly seen from this data, increasing the concentration ofsingle crystal silicon carbide whiskers accelerated the temperatureincrease. Thus, the dielectric loss tangent increased with increasingsingle crystal silicon carbide whisker concentration.

Example 3 Heating of a Low Dielectric Solid

Alumina is a ceramic material that has a very low dielectric losstangent in the RF and microwave spectra. Therefore, it does notefficiently convert this energy into heat. Ceramic composites (compositematerial of the invention) were made by combining alumina with thesingle crystal silicon carbide whiskers of Example 1. These compositeswere then hot-pressed into a ceramic composite material of theinvention. This ceramic composite was subjected to a microwave field of2.45 GHz at 1000 watts. The temperature of the ceramic was recorded withtime. Results are shown in FIG. 3.

As can be clearly seen, single crystal silicon carbide whiskers formedcomposite materials of the invention with alumina and heated rapidlycompared with the microwave-transparent matrix material (alumina), andthe rate of temperature increase was made larger by a greaterproportion, measured as wt percent, of single crystal silicon carbidewhiskers in the composite material of the invention.

Example 4 Measurement of Dielectric Properties of Porous Powders

Single crystal silicon carbide whiskers of Example 1 were again added toalumina, a low dielectric loss solid. In this example, the singlecrystal silicon carbide whiskers were added at increasing weightpercentages, with the goal of making composite materials of theinvention with increasing dielectric loss tangent concurrent withincreasing concentration of single crystal silicon carbide whiskers. Thedielectric loss tangents of the blended, porous powders were measured.Metal probes were placed into the powders, and then the porous powderswere subjected to a frequency network analyzer at 915 MHz. The networkanalyzer determined the relative dielectric constant and the losstangents of these powders.

Four powders were tested:

(1) Pure alumina, with 0% by weight single crystal silicon carbidewhiskers;

(2) Alumina with 7.5% by weight single crystal silicon carbide whiskers;

(3) Alumina with 15% by weight single crystal silicon carbide whiskers;and

(4) Alumina with 25% by weight single crystal silicon carbide whiskers.

The resultant dielectric loss tangents are summarized in Table 3 belowand are illustrated graphically in FIG. 4:

TABLE 3 Wt percent single crystal silicon carbide whiskers in aluminaDielectric loss tangent 0 0.0007 7.5 0.073 15 0.171 25 0.259

This test showed that pure alumina has a very low loss tangent. Itsability to convert the applied 915 MHz field into heat was very small.The effect of the single crystal silicon carbide whiskers in thecomposite materials of the invention was clear: the loss tangentincreased from 0.0007 for pure alumina up to 0.259 for compositematerial of the invention comprising alumina powder containing 25%single crystal silicon carbide whiskers. The silicon carbide singlecrystal dramatically increased the loss tangent by a factor of about37,000%.

Example 5 Comparison of Single Crystal Silicon Carbide Whiskers Used toMake Composite Material of the Invention to Other Forms of SiliconCarbide

The ability to increase temperature upon exposure to microwave energy isa direct reflection of the dielectric loss tangent of the compositionwhen all other variables are kept constant. The ability of finepolycrystalline silicon carbide particulate to increase temperature uponexposure to microwave energy was compared to the ability of singlecrystal silicon carbide whiskers as used in Example 1 and to singlecrystal silicon carbide fibrils to increase temperature under the sameconditions. These single crystal silicon carbide fibrils were the samediameter as the single crystal silicon carbide whiskers of Example 1,but were approximately 20 microns in length, as compared with the 9micron length of the single crystal silicon carbide whiskers of Example1.

The identity and manufacturer of each particulate silicon carbide isidentified in Table 5 below. To compare the materials, 100 grams of eachmaterial was separately placed into an alumina crucible and thenseparately subjected to a 2.45 GHz microwave field at 1000 watts.

The temperature rise of each material then was determined. Table 4summarizes and FIG. 5 illustrates the result. Surprisingly, and inaccordance with the invention, the single crystal silicon carbidewhiskers demonstrate much higher conversion of microwave radiation intoheat. In addition, the longer crystal size of the single crystal siliconcarbide whiskers used in the invention provided more efficientconversion of microwave energy to heat.

TABLE 4 Temperature Silicon Carbide Source Incr., ° C. Single crystalsilicon carbide fibrils, 20 micron crystal 218 Single crystal siliconcarbide whiskers, 9 micron crystal 164 Sika 1500 (Saint Gobain,Lillesand, Norway) 41 Betarundum (Ultrafine) (Ibiden Company, Japan) 32Densic Ultrafine (B-1) (Showa Denko, Japan) 35 DensicUltrafine (A-1)(Showa Denko, Japan) 33 Carbogran (UF 15) (Lonza Inc, Fairlawn, NJ) 38Carbogran (UF 10) (Lonza Inc., Fairlawn, NJ) 36 Carbogran (05) (LonzaInc., Fairlawn, NJ) 43 Crystolon (Norton, Worcester, MA) 64 Hermann C.Starck (B 10) (Hermann Starck, Newton, MA) 35 Hermann C. Starck (A 10)(Hermann Starck, Newton, MA) 37

In Examples 6-8, single crystal silicon carbide whiskers having anaverage diameter of 0.6 microns and an average length of 10 microns wereblended with alumina powder to form a composite material of theinvention. A mass of each composite material of the invention shown intables below was heated in a single magnetron microwave oven at afrequency of 2.45 GHz and power of about 1000 watts.

Each powder was heated five times; the average of the tests is shown inthe tables. Each heating period lasted 600 seconds, or until thetemperature reached 1300° C. There were some differences in heating ratefor each trial with a specific mass. Whereas the average data in thetables may not exceed 1300° C., each trial of the 15 and 25 wt percentsingle crystal silicon carbide whiskers samples attained a temperatureof at least 1300° C.

Those skilled in the art know that one challenge in microwave heating isfield non-uniformity, as described above. Typical microwave wavelengthsvary from 10 cm to 12 cm in length. Thus, these wavelengths are long,and can result in a non-uniform field. A specimen may experience highflux rates of radiation in 1-2 cm zone. However, outside this hot zoneby a distance as small as 1-2 cm, the flux rate can be significantlylower. Therefore, uneven heat rates experienced in these Examples isbelieved to have been caused by non-uniform field distribution.

Various techniques can be utilized to improve field uniformity.Techniques include: (1) using multiple magnetrons to inject microwaveradiation into the chamber at different locations, (2) waveguides andwave-splitting devices to control microwave directionality, (3) metalreflectors, fans, and “stirrers” to disperse the microwave radiation,and (4) turntables or other devices to move the specimen in the field,thereby distributing the flux over the specimen.

Use of these techniques generally improves the uniformity of heatingrates. However, the oven used in the experiments was a single magnetronsystem, and therefore had distinct field non-uniformity. Therefore,certain precautions were taken to improve accuracy and reliability ofthe results of this single magnetron system. Prior to conducting heatingrate tests, the distribution of the microwave field was mapped.So-called “hot spots” and “cold spots” were identified. Specimens formeasurement then were placed directly in the areas of high energy flux,i.e., the “hot spots.” For the most part, this approach deliversrepeatable results. However, if the specimen was placed slightly offlocation, or if the specimen itself distorts the energy field, thenrelatively minor changes in heating rates will have been experienced.

Therefore, to ensure the data is high-quality and repeatable, theheating rate tests were repeated five times. In some cases, slowerheating rates were observed. Although the inventors do not wish to bebound by theory, it is believed that these differences in heating rateswere due to reasons mentioned. Nevertheless, the average heating ratesclearly demonstrated the utility of the invention. In addition, thisapproach clearly demonstrated the utility of the invention in ‘realworld’ applications where field non-uniformity is a reality.

Example 6 Heating Rate of Composite Material of the Invention in aCrucible

The crucible used to contain the composite material of the invention hada diameter of 38.1 mm and a height of 89.2 mm. The composite materialsof the invention were blended powders of alumina and single crystalsilicon carbide whiskers comprising 25, 15, and 7.5 wt percent singlecrystal silicon carbide whiskers. The powder charges to the crucible hadaverage weights of 74.8 grams, 79.5 grams, and 72.2 grams, respectively.The composite materials of the invention occupied 76.0 vol percent, 91.1vol percent, and 91.9 vol percent of the volume of the crucible,respectively, for the 25 wt percent, 15 wt percent, and 7.5 wt percentcomposite materials of the invention.

FIG. 6 depicts the average heating rate for each sample. Each compositematerial of the invention comprised single crystal silicon carbidewhiskers with alumina. As can be seen, the composite material of theinvention comprising 25 wt percent single crystal silicon carbidewhiskers heated most quickly. Also, both the 15 and 7.5 wt percentcomposite material of the invention heated to temperatures well abovetemperatures typically achieved with polycrystalline silicon carbideparticulate as the microwave receptor.

Example 7 Comparative Example

The heating ability of composite material of the invention comprising 15and 25 wt percent single crystal silicon carbide whiskers of Example 6were compared to comparative samples prepared with polycrystallinesilicon carbide particles having an average particle size of less than 1micron with alumina. Loose powder was heated for 600 seconds in the samecrucible used in Example 6. The average weight and volume percentage ofthe crucible occupied by the 25 wt percent and 15 weight percent sampleswere 92.4 grams, 77.5 vol percent, and 93.5 grams, 77.4 vol percent,respectively. As can be seen in FIG. 7, the 25 percent polycrystallinesilicon carbide heated to 1016° C. in 600 seconds, but the lowerconcentration did not reach this temperature.

From this information, it can be seen that composite material of theinvention comprising single crystal silicon carbide whiskers heated muchmore quickly, and to a higher temperature, than the same matrix material(alumina) blended with polycrystalline silicon carbide particles.

Example 8 Rods Comprising Composite Material of the Invention

Composite material of the invention comprising 7.5 and 15 wt percentsingle crystal silicon carbide whiskers, with the remainder alumina,were extruded into a rod shape, then sintered into a fused ceramic rod.The characteristic dimensions and weights of the rods are shown in Table5.

Four rods were heated at the same time in the crucible of Example 6 andin accordance with the method of Example 6. Five trials were carriedout, with the average depicted in FIG. 8.

TABLE 5 Percentage single crystal silicon carbide whiskers 7.5 15 Roddiameter, cm 1.42 1.45 Rod length, cm 10.3 6.9 Rod weight, grams 60.442.5

As expected, the rods having the greater concentration of single crystalsilicon carbide whiskers heated more quickly. However, comparison withthe same tests carried out in a crucible on loose powder having the sameconcentrations of single crystal silicon carbide whiskers in Example 6indicated that rod heats faster at 7.5 wt percent single crystal siliconcarbide whiskers, but the powder heated faster at 15 wt percent singlecrystal silicon carbide whiskers. The skilled practitioner recognizesthat the rods, which are less porous than the powder blend, should heatmore quickly than loose powder having the same single crystal siliconcarbide whiskers and fibrils concentration. In these trials, thethermocouple used to measure the temperature was not connected directlyto the rods because this caused sparking. Although the inventors do notwish to be bound by theory, it is believed that this arrangementresulted in differences between temperature measurement for the rods andthe powder. In any event, the data showed that composite material of theinvention in rod form heats quickly, efficiently, and to hightemperature.

Example 9

A ceramic mixture was produced by blending 7.5% silicon carbide whiskerswith high purity alumina (>99.9% alumina by weight). Sintering aids ofmagnesia and yttria were also added at levels up to 5 volume percent.The mixture was milled until a uniform dispersion of silicon carbidewhisker in the ceramic blend was attained.

This ceramic mixture was then compounded into a paste to enable furtherprocessing via extrusion. To produce the paste, the ceramic powders weremixed with water, mineral oil, and carboxymethyl cellulose. Theresultant paste was 88% solids by weight. Other water-soluble polymerscan be used, including linear, branched, and crosslinked polymers, andwell as cationic, anionic and nonionic versions, can be used. Thesewater-soluble polymers include hydroxyethyl cellulose, carageenan,xanthan gum, poly-acrylates, poly vinyl alcohols, and poly acrylamides,to name a few.

Once a paste was formed, the mixture was extruded through a die to formlinear rods. These rods were cut to a desired length of 13 inches. Theserods were then dried, and placed in a dewax oven at 500° C. to burn outthe binder and remove all residual organics. After drying and dewaxing,the product had a green density of 63% of theoretical. The product wassintered at 1600° C. for 24 hours in a nitrogen atmosphere. The productwas cool slowly to room temperature and removed from the sinter kiln,resulting in microwave absorbent rod. The finished rod density was >96%of theoretical.

A rack with supporting members was fabricated from stainless steelsimilar to the rack as shown in FIG. 10A. A unitary piece of stainlesssteel was used to avoid arcing by the frame itself. The microwaveabsorbent rods were placed in this rack, and the rods were locked intoplace. The final result was a microwave absorbent and heatable rack withthe frame made out of stainless steel and the microwave heatableelements from the ceramics as manufactured above.

Example 10 Melting Aluminum Powder

A blend of 75 wt percent alumina and 25 wt percent single crystalsilicon carbide whiskers having an average diameter of 0.6 microns andan average length of 9 microns was prepared to form composite materialof the invention. This composite material of the invention was placed inthe void between two zirconia crucibles of different diameter andheight. Metallic aluminum was placed in the smaller crucible, and theentire mass was heated in a microwave oven at a frequency of 2.45 GHzand power of about 1000 watts.

The larger zirconia crucible had a diameter of 69.3 mm and a height of105.3 mm. The smaller crucible had a diameter of 38.1 mm and a height of82.6 mm. A quantity of 75.1 grams of the composite material of theinvention in powder form was placed in the larger crucible, and thesmall crucible nested inside the larger crucible. The smaller cruciblewas pushed down into the powder composite material of the invention sothat the composite material of the invention came up the sides thesmaller crucible.

Five grams of metallic aluminum were placed inside the smaller crucible,which was itself surrounded by the powder composite material of theinvention. The apparatus was insulated with one inch thick,high-temperature fibrous alumina-silicate insulation, and the entiremass was heated, with the objective of maintaining a metal temperatureof 1000-1050° C. to effect melting of the aluminum. As can be seen inFIG. 9, the temperature reached 1007° C. after 750 seconds. Thereafter,the temperature was maintained essentially within the range of1000-1050° C. thereafter.

As the aluminum metal melted, a solid/liquid equilibrium formed. At theend of the heating period, the entire system was cooled to roomtemperature. The aluminum had formed a solid mass, thus showing thatcomposite material of the invention can be used to heat, soften, anneal,and melt metals.

While the invention has been described with respect to specific examplesincluding presently preferred modes of carrying out the invention, thoseskilled in the art will appreciate that there are numerous variationsand permutations of the above described systems and techniques that fallwithin the spirit and scope of the invention as set forth in theappended claims.

We claim:
 1. A microwave absorbing and heatable food processing articlecomprising composite material that increases in temperature uponexposure to electromagnetic radiation, said composite materialcomprising single crystal silicon carbide whiskers, fibrils, or a blendthereof, in a matrix material that is substantially transparent toelectromagnetic radiation, wherein the composite material has adielectric loss tangent which is at least 100 times greater than that ofthe matrix material.
 2. The article of claim 1 wherein the compositematerial comprises single crystal silicon carbide whiskers having adiameter between about 0.2 and about 10 microns and an aspect ratio ofbetween about 10 and
 25. 3. The article of claim 1 wherein the compositematerial comprises silicon carbide fibrils having a diameter betweenabout 4 and about 20 microns and a length of at least about 50 microns.4. The article of claim 1 wherein the single crystal silicon carbidewhiskers and fibrils further comprise a dopant.
 5. The article of claim4 wherein the dopant is nitrogen.
 6. The article of claim 1 wherein thearticle browns food contained therein when the article is exposed tomicrowave radiation.
 7. The article of claim 1 wherein the matrixmaterial comprises a polymer.
 8. The article of claim 1 wherein thematrix material comprises a polymeric film.
 9. The article of claim 1wherein the matrix material comprises a fluorocarbon polymer.
 10. Thearticle of claim 1 wherein the matrix material comprises a thermoplasticpolymer.
 11. The article of claim 1 wherein the matrix materialcomprises a crosslinked (thermoset) polymer.